Comparative effect of inorganic and organic selenocyanate derivatives in mammary cancer chemoprevention

Small-molecule organoselenocyanates: Recent developments toward synthesis, anticancer, and antioxidant activities

Cellular redox homeostasis is very important for the overall cellular development, function, and oxidative stress often disrupts the process. Small-molecule organoselenium compounds exert key roles in maintaining the redox homeostasis during oxidative stress and cancer owing to their notable antioxidant activities. Among different organoselenium compounds, small-molecule organoselenocyanates have attracted much research attention due to their synthetic utilities and therapeutic potentials. Therefore, the development of convenient synthetic methodologies to different classes of organoselenocyanates from various precursors was explored over the years as useful synthetic building blocks. Additionally, considering their inherent redox and antioxidant properties, the development of biologically relevant organoselenocyanates upon their conjugation with the existing drugs and natural products has been chosen for enhancing the drug potencies and in ameliorating the drug-induced side-effects. In the present report, we have discussed some of the very recent and relevant developments on these aspects in a very concise manner.

Genetic and Enzymatic Characteristics of CYP2A13 in Relation to Lung Damage

Cytochrome P450 2A13 is an omitted brother of CYP2A6 that has an important role in the drug metabolism of liver. Due to extrahepatic expression, it has gained less attention than CYP2A6, despite the fact that it plays a significant role in toxicant-induced pulmonary lesions and, therefore, lung cancer. The purpose of this mini-review is to summarize the basic knowledge about this enzyme in relation to the substrates, inhibitors, genetic polymorphisms, and transcriptional regulation that are known so far (September 2021).

Diselenides and Selenocyanates as Versatile Precursors for the Synthesis of Pharmaceutically Relevant Compounds

Organoselenium chemistry has undergone extensive development during the past decades, mostly due to the unique chemical properties of organoselenium compounds that have been widely explored in a number of synthetic transformations, as well as due to the interesting biological properties of these compounds. Diselenides and selenocyanates constitute the promising classes of organoselenium compounds that possess interesting biological effects and that can be used in the preparation of other selenium compounds. The combination of diselenide and selenocyanate moieties with other biologically relevant molecules (such as heterocycles, steroids, etc.) is a way for the development of compounds with promising pharmaceutical potential. Therefore, the aim of this review is to highlight the recent achievements in the use of diselenides or selenocyanates as precursors for the synthesis of pharmaceutically relevant compounds, preferentially compounds with antitumor and antimicrobial activities.

Effect of organo-selenium anticancer drugs on nitrite induced methemoglobinemia: A spectroscopic study

Selenium containing drugs like selenomethionine, selenocystine, selenourea and methylseleninic acid are reported to exhibit potential anticancer effect. However, these anticancer drugs may exert adverse effects when used over a prolonged period. Little is known about the interaction of these selenium containing drugs with the vital erythroid protein hemoglobin. In this work a comparative study of the interaction of organo-selenium drugs with hemoglobin and heme moiety has been performed using different spectroscopic techniques to find out their role on drug induced methemoglobinemia. We found that though these selenium containing drugs have similar binding affinity towards hemoglobin, they have differential interactions with the heme group. Isothermal calorimetric titration study showed that selenourea has the lowest binding affinity (K_d 19.28 μM) towards HbA as compared to other drugs, selenomethionine, selenocystine and methylseleninic acid (K_d 7.69 μM, 4.88 μM and 10.5 μM at 37 °C respectively). This result is also supported by the molecular docking study. Methylseleninic acid was found to have detrimental effects on nitrite induced methemoglobinemia, a hematological disorder caused due to excessive conversion of Fe²⁺ to Fe³⁺ in hemoglobin. Hence the results of the study would help to develop a better insight on the mechanism of action and anticipate the toxicity of these drugs which require further optimization before their actual use in the treatment of cancer.

Increased Transglutaminase 2 and GLUT-1 Expression in Breast Tumors not Susceptible to Chemoprevention with Antioxidants

Goals

Expression of GLUT-1 and transglutaminase 2 is increased in aggressive breast cancer, whereas claudin-1, which is expressed in normal tissues, is absent in such tumors. This experimental study was undertaken to establish the aggressiveness and prognosis of DMBA-induced mammary tumors in female Wistar rats based on the assessment of these markers.

Materials and methods

The rats were divided into two groups, a control group (n = 70) and a chemoprevention group (n = 70). Breast tumors were induced in both groups by administration of 7,12-dimethylbenz[a]anthracene (DMBA). The chemoprevention group also received alpha-tocopherol and a solution of micronutrients containing ascorbic acid and selenium. Neoplastic lesions of both groups were randomly selected for immunohistochemical assessment of the expression of GLUT-1, transglutaminase 2 and claudin-1.

Results

A higher proportion of mammary tumors expressed GLUT-1 and transglutaminase 2 in the chemoprevention group. Claudin-1 expression was absent in all tumors of both groups.

Conclusions

These results are suggestive of increased aggressiveness of tumors not susceptible to chemoprevention by the agents used in this study.

Potent anti-proliferative activities of organochalcogenocyanates towards breast cancer

The synthesis of benzylic and mesitylenic organochalcogenocyanates has been described and the compounds have been studied for their anti-proliferative activities in breast cancer cells (MDA-MB-231, MCF-7 and T-47D).

Analysis of Selenium Levels in Osteosarcoma Patients and the Effects of Se-Methylselenocysteine on Osteosarcoma Cells In Vitro

The form of selenium appears to be important for preventing cancer in humans. Here, we evaluated selenium levels in the serum and bone tissue samples from osteosarcoma patients using atomic absorption spectrometry. The in vitro effects of Se-methylselenocysteine (Se-MSC) on growth, cell cycle status, and apoptosis of osteosarcoma cells were assessed using the WST-1 assay, Hoechst 33342/propidium iodide staining, and flow cytometry, respectively. In osteosarcoma cases, the mean serum selenium levels in osteosarcoma tissue and normal bone were 0.08 mg/kg and 0.03 mg/kg, respectively (P < 0.05). Serum selenium levels in osteosarcoma and non-osteosarcoma cases were 0.09 mg/L and 0.08 mg/L, respectively (P > 0.05). Se-MSC-treated MG63 cells showed altered cellular morphology, decreased viability in a time- and dose-dependent manner, and an increase in the sub-G1 cell population. Se-MSC also downregulated Bcl-2 expression and upregulated Bax. Se-MSC inhibited the proliferation of the drug-resistant osteosarcoma cell line Saos-2/MTX300 and enhanced the inhibitory effect of pirarubicin on MG63 cells. Our data demonstrate that selenium levels are significantly higher in osteosarcoma tissue than normal bone tissue in osteosarcoma patients. The results also support the anticancer effects of Se-MSC in osteosarcoma. Further development of Se-MSC as an ancillary chemotherapy agent in osteosarcoma is warranted.

A novel chemopreventive mechanism of selenomethionine: enhancement of APE1 enzyme activity via a Gadd45a, PCNA and APE1 protein complex that regulates p53-mediated base excision repair

Organic selenium compounds have been documented to play a role in cancer prevention. Our previous study showed that selenomethionine (SeMet) induces p53 activation without genotoxic effects including apoptosis and cell cycle arrest. In this study, we investigated the mechanism by which organic selenium compounds promote p53-mediated base excision repair (BER) activity. Our data demonstrated for the first time that the interaction between growth arrest and DNA damage-inducible protein 45A (Gadd45a), which is a p53-activated downstream gene, and two BER-mediated repair proteins, proliferating cell nuclear antigen (PCNA) and apurinic/apyrimidinic endonuclease (APE1/Ref-1), was significantly increased in a p53-dependent manner following treatment with organic selenium compounds. Furthermore, we observed that the activity of APE1 was significantly increased in a p53-dependent manner in response to the organic selenium compounds. These results suggest that BER activity is dependent on wild-type p53 activity and is mediated by the modulation of protein interactions between Gadd45a and repair proteins in response to organic selenium compounds. We propose that p53-dependent BER activity is a distinct chemopreventive mechanism mediated by organic selenium compounds, and that this may provide insight into the development of effective chemopreventive strategies against various oxidative stresses that contribute to a variety of human diseases, particularly cancer.

Influence of novel naphthalimide-based organoselenium on genotoxicity induced by an alkylating agent: the role of reactive oxygen species and selenoenzymes

OBJECTIVE

The protection conferred by a series of synthetic organoselenium compounds against genotoxicity and oxidative stress induced by a reference mutagen cyclophosphamide (CP) was assessed.

METHOD

Genotoxicity was induced in mice by CP treatment (25 mg/kg b.w.) for 10 consecutive days. Organoselenium compounds (3 mg/kg b.w.) were administered orally in a concomitant and pretreatment schedule. DNA damage in peripheral blood lymphocytes and frequency of chromosomal aberration in the bone marrow cells were measured. Liver tissues were collected for analysis of the activity of antioxidant and detoxifying enzymes, lipid peroxidation (LPO) level, glutathione content, and histopathology.

RESULTS

Exposure to CP not only led to a significant increase in the percent of chromosomal aberration and DNA damage, but also enhanced generation of hepatic reactive oxygen species (ROS) and LPO level. The organoselenium compounds demonstrated marked functional protection against CP-induced genotoxicity. DNA damage and chromosomal aberration along with ROS generation were attenuated in the organoselenium-treated mice compared with the CP-treated control mice. CP caused marked depression in the activities of the selenoenzymes (glutathione peroxidase (GPx) and thioredoxin reductase (TRxR)) and other detoxifying and antioxidant enzymes, while treatment with organoselenium compounds restored all these activities towards normal.

DISCUSSION

The protective effect of these compounds may be primarily associated with the improvement of the activity of antioxidant and detoxifying enzymes (including the selenoenzymes, GPx, and TRxR) that are known to protect the DNA and other cellular components from oxidative damage.

Spectral modification and catalytic inhibition of human cytochromes P450 1A1, 1A2, 1B1, 2A6, and 2A13 by four chemopreventive organoselenium compounds

Several organoselenium compounds including benzyl selenocyanate (BSC), 1,2-phenylenebis(methylene)selenocyanate (o-XSC), 1,3-phenylenebis(methylene)selenocyanate (m-XSC), and 1,4-phenylenebis(methylene)selenocyanate (p-XSC) have been shown to prevent cancers caused by polycyclic aromatic hydrocarbons (PAHs) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in experimental animals; these chemical carcinogens are activated by human P450 1 and 2A family enzymes, respectively, to carcinogenic metabolites. In this study, we examined whether these selenium compounds interact with and inhibit human P450 1 and 2A enzymes in vitro. Four organoselenium compounds induced reverse Type I binding spectra with P450 1A1, 1A2, and 1B1 and Type I binding spectra with P450 2A6 and 2A13. The spectral dissociation constants (K(s)) for the interaction of P450 1B1 with these chemicals were 3.6-5.7 μM; the values were lower than those with seen with P450 1A1 (19-30 μM) or 1A2 (6.3-13 μM). The K(s) values for Type I binding of P450 2A13 with m-XSC and BSC were both 0.20 μM; the values were very low compared to those for the interaction of P450 2A6 with m-XSC (5.7 μM) and BSC (2.0 μM). Four selenium compounds directly inhibited 7-ethoxyresorufin O-deethylation activities catalyzed by P450 1A1, 1A2, and 1B1 with IC(50) values <1.0 μM, except for the inhibition of P450 1A2 by BSC (1.3 μM). Coumarin 7-hydroxylation activities of P450 2A13 were more inhibited by four selenium compounds than those of P450 2A6, with IC(50) values of 0.22-1.4 μM for P450 2A13 and 2.4-6.2 μM for P450 2A6. Molecular docking studies of the interaction of four organoselenium compounds with human P450 enzymes suggest that these chemicals can be docked into the active sites of these human P450 enzymes and that the sites of the selenocyanate functional groups of these chemicals differ between the P450 1 and 2A family enzymes.

Glutathione peroxidases in different stages of carcinogenesis

Cancer cells produce high amounts of reactive oxygen species (ROS) and evade apoptosis. Hydroperoxides support proliferation, invasion, migration and angiogenesis, but at higher levels induce apoptosis, thus being pro- and anti-carcinogenic. Accordingly, glutathione peroxidases (GPxs) regulating hydroperoxide levels might have dual roles too. GPx1, clearly an antioxidant enzyme, is down-regulated in many cancer cells. Its main role would be prevention of cancer initiation by ROS-mediated DNA damage. GPx2 is up-regulated in cancer cells. GPx1/GPx2 double knockout mice develop colitis and intestinal cancer. However, GPx2 knockdown cancer cells grow better in vitro and in vivo probably reflecting the physiological role of GPx2 in intestinal mucosa homeostasis. GPx2 counteracts COX-2 expression and PGE(2) production, which explains its potential to inhibit migration and invasion of cultured cancer cells. Overexpression of GPx3 inhibits tumor growth and metastasis. GPx4 is decreased in cancer tissues. GPx4-overexpressing cancer cells have low COX-2 activity and tumors derived therefrom are smaller than from control cells and do not metastasize. Collectively, GPxs prevent cancer initiation by removing hydroperoxides. GPx4 inhibits but GPx2 supports growth of established tumors. Metastasis, but also apoptosis, is inhibited by all GPxs. GPx-mediated regulation of COX/LOX activities may be relevant to early stages of inflammation-mediated carcinogenesis.

Differential sensitivity of various human tumour-derived cell types to apoptosis by organic derivatives of selenium

Selenium is an important trace element with anti-cancer properties. In the present study, the apoptosis-inducing effects of organic selenium derivatives, namely methyl-L-selenocysteine and selenomethionine, were evaluated in vitro on human tumour-derived cell lines from breast, liver, colon, brain, skin and a non-tumorigenic line of epithelial origin. Apoptosis was assessed by cell-death detection immunoassay on cytoplasmic cell lysates. Breast carcinoma cells were highly sensitive to the organic selenium compounds, manifesting apoptosis at concentrations as low as 0.113 microm (0.0205 microg/ml) selenium. By contrast, non-tumorigenic mammary epithelial cells displayed poor sensitivity to selenium, requiring a substantially high concentration of the trace element of 87.9 microm (16.0 microg/ml). The cell lines derived from hepatoma and neuroblastoma showed intermediate sensitivity, with colon carcinoma cells manifesting the lowest sensitivity to the trace element. These results indicate intrinsic differences in the sensitivity of human tumour derivatives to selenium-mediated apoptosis, providing experimental support for the development of organic selenium compounds as anti-neoplastic agents against solid tumours displaying selective apoptotic sensitivity to these compounds.

Superior in vivo inhibitory efficacy of methylseleninic acid against human prostate cancer over selenomethionine or selenite

Methylselenol has been implicated as an active anticancer selenium (Se) metabolite. However, its in vivo efficacy against prostate cancer (PCa) has yet to be established. Here, we evaluated the growth inhibitory effects of two presumed methylselenol precursors methylseleninic acid (MSeA) and Se-methylselenocysteine (MSeC) in comparison with selenomethionine (SeMet) and selenite in DU145 and PC-3 human PCa xenografts in athymic nude mice. Each Se was given by a daily single oral dose regimen starting the day after the subcutaneous inoculation of cancer cells. We analyzed serum, liver and tumor Se content to confirm supplementation status and apoptosis indices and tumor microvessel density for association with antitumor efficacy. Furthermore, we analyzed lymphocyte DNA integrity to detect genotoxic effect of Se treatments. The data show that MSeA and MSeC exerted a dose-dependent inhibition of DU145 xenograft growth and both were more potent than SeMet and selenite, in spite of less tumor Se retention than in the SeMet-treated mice. Selenite treatment increased DNA single-strand breaks in peripheral lymphocytes, whereas the other Se forms did not. Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) and cleaved caspase-3 indices (apoptosis) from MSeC-treated tumors were higher than tumors from control mice or MSeA-treated mice, whereas the microvessel density index was lower in tumors from MSeA-treated mice. In the PC-3 xenograft model, only MSeA was growth inhibitory at a dose of 3 mg/kg body wt. In summary, our data demonstrated superior in vivo growth inhibitory efficacy of MSeA over SeMet and selenite, against two human PCa xenograft models without the genotoxic property of selenite.

Selenised casein protects against AOM-induced colon tumors in Sprague Dawley rats

Selenium (Se) has been shown to be protective against cancers in animal models at concentrations exceeding those considered essential for normal nutritional requirements. Organic forms of Se provided as dairy proteins were obtained from cows fed diets supplemented with yeast Se. The casein extracted from milk was found to contain approximately half the Se of the Se-enriched milk. This casein was included in a semi-purified AIN rodent diet so as to provide 1 ppm Se and 25% protein and was compared with AIN diets containing no added Se (control, 0.05 ppm), 1 ppm and 4 ppm Se as selenised yeast (Sel-Plex) Their influence on colon tumor expression was examined in rats induced with azoxymethane, the diets being introduced post-induction. The selenised casein diet at this concentration was effective in reducing colon tumor incidence (by 29%) and burden (decreased 52%, P < 0.05) relative to the control in rats 26 wk post-induction. Selenised yeast, when added at similar (1 ppm) and increased Se concentration (4 ppm), did not influence significantly colon tumor expression. However, in a second study, with Se yeast providing Se at 1 ppm, 4 ppm, and 8 ppm throughout the experiment, a significant reduction in tumors was observed with 8 ppm Se (colon tumor incidence was 15% lower and colon tumor burden was 35% lower, P < 0.05). However this was associated with a significantly lower body weight in the rats (down 10.5%, P < 0.05) indicating a possible disturbance with normal energy intake or metabolism. The form in which Se is presented in the diet may influence significantly its bioavailability and/or anticancer potential at given concentrations within a safe range. The efficacy of selenised casein and indeed other potential dietary sources deserve further investigation with regard to their ability to prevent colon tumors at concentrations considered safe in the diet.

Xenobiotic-metabolizing enzymes involved in activation and detoxification of carcinogenic polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental carcinogens and metabolized by a variety of xenobiotic-metabolizing enzymes such as cytochrome P450 (P450 or CYP), epoxide hydrolase, glutathione transferase, UDP-glucuronosyltransferase, sulfotransferase, NAD(P)H quinone oxidoreductase 1, and aldo-keto reductase. These enzymes mainly participate in the conversion of PAHs to more polar and water-soluble metabolites, and the resultant metabolites are readily excreted from the body. However, during the course of metabolism, a variety of unstable and reactive intermediates of PAHs are formed, and these metabolites attack DNA, causing cell toxicity and transformation. P450s and epoxide hydrolase convert PAHs to proximate carcinogenic metabolites, PAH-diols, and these products are further metabolized by P450s to ultimate carcinogenic metabolites, PAH diol-epoxides, or by aldo-keto reductase to reactive PAH o-quinones. PAHs are also activated by P450 and peroxidases to reactive radical cations that bind covalently to DNA. The oxygenated and reactive metabolites of PAHs are usually converted to more polar and detoxified products by phase II enzymes. Inter-individual differences exist in levels of expression and catalytic activities of a variety of enzymes that activate and/or detoxify PAHs in various organs of humans and these phenomena are thought to be critical in understanding the basis of individual differences in response to PAHs. Factors affecting such variations include induction and inhibition of enzymes by diverse chemicals and, more importantly, genetic polymorphisms of enzymes in humans.

Cancer chemoprevention by garlic and garlic-containing sulfur and selenium compounds

As early as 1550 B.C., Egyptians realized the benefits of garlic as a remedy for a variety of diseases. Many epidemiological studies support the protective role of garlic and related allium foods against the development of certain human cancers. Natural garlic and garlic cultivated with selenium fertilization have been shown in laboratory animals to have protective roles in cancer prevention. Certain organoselenium compounds and their sulfur analogs have been identified in plants. Organoselenium compounds synthesized in our laboratory were compared with their sulfur analogs for chemopreventive efficacy. Diallyl selenide was at least 300-fold more effective than diallyl sulfide in protecting against 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary adenocarcinomas in rats. In addition, benzyl selenocyanate inhibited the development of DMBA-induced mammary adenocarcinomas and azoxymethane-induced colon cancer in rats and benzo[a]pyrene-induced forestomach tumors in mice. The sulfur analog, benzyl thiocyanate, had no effect under the same experimental conditions. Furthermore, we showed that 1,4-phenylenebis(methylene)selenocyanate, but not its sulfur analog, significantly inhibited DMBA-DNA adduct formation and suppressed DMBA-induced mammary carcinogenesis. Collectively, these results indicate that structurally distinctive organoselenium compounds are superior to their corresponding sulfur analogs in cancer chemoprevention. Additionally, synthetic aromatic selenocyanates are more effective cancer chemopreventive agents than the naturally occurring selenoamino acids. Because plants are capable of utilizing selenium in a manner similar to that in sulfur assimilation pathways, future studies should aim at determining whether, under appropriate conditions, these potent cancer chemopreventive synthetic selenium compounds can be synthesized by garlic and related allium foods.

Hepatic chemoprotective enzyme responses to 2-substituted selenazolidine-4(R)-carboxylic acids

In epidemiology and human supplementation studies, as well as many animal models, selenium has shown antitumorigenic activity. The mechanism of action, however, has not been satisfactorily resolved. Selenium supplementation affects many enzymes in addition to those where selenocysteine is an essential component. Such enzymes include cytoprotective detoxifying enzymes, and the regulation of these enzymes by a set of 2-substituted selenazolidine-4(R)-carboxylic acids (SCAs) has been investigated. Following seven consecutive daily doses of these prodrugs of L-selenocysteine, changes in hepatic enzyme activities and/or mRNA levels of glutathione transferase (GST), microsomal epoxide hydrolase (mEH), NAD(P)H-quinone oxidoreductase (NQO), UDP-glucuronosyltransferase (UGT), glutathione peroxidase (GPx), and thioredoxin reductase (TR) have been observed. Among the enzymes examined, UGTs and GPx were found to be the least affected. Among the compounds, 2-oxoSCA produced the most changes and 2-phenylSCA produced the least, none. For no two compounds was the pattern of changes identical, and for a single compound, few changes were reproduced in common by the two routes of administration investigated. In general, more changes were elicited following intraperitoneal (i.p.) administration than with the intragastric (i.g.) route. This dominance was typified by 2-butylSCA and 2-cyclohexylSCA where enzyme activity elevations (TR and mEH with both, NQO with 2-butylSCA) were seen only with the i.p. route. With 2-oxoSCA, however, GST, TR, and NQO activities were found to be elevated independent of route. Only with GST (both routes) and TR (i.p. route), elevations in mRNAs accompanied the 2-oxoSCA elicited elevations of activities at the time of sacrifice. For some enzymes, most notably mEH with compounds administered i.p., elevations in mRNAs were not manifest as increased enzyme activity. Thus, although constituting a closely related series of compounds, each 2-substituted SCA produced its own unique pattern of changes, and for most members, changes were predominant following i.p. administration.

The inhibitory effect of sodium selenite on N-nitrosodiethylamine-induced and phenobarbital promoted liver tumourigenesis in rats based on the modulation of polyamine levels

In the present study, we have evaluated the effects of dietary selenite (Se) on polyamine levels and its influence on N-nitrosodiethylamine (DEN) initiated and Phenobarbital (PB) promoted in rat liver carcinogenesis. Dietary selenite at a concentration of 4 ppm (through drinking water) was administered in rats either before initiation (4 weeks), or during promotion (16 weeks) and entire experimental period (20 weeks). Male Wistar strain of albino rats was treated with single intra peritoneal dose of DEN (200 mg kg(-1) body weight), after 2 weeks the carcinogenic effect was promoted by PB (0.05%; through diet). Alpha fetoprotein (AFP) was investigated after the 20th-week of experimental period. Selenite-treated animals markedly reduced the AFP during the time of pre-selenite [before initiation (4 weeks)] and entire experimental period (20 weeks), administration rather than the promotion period. This infers that anticancer property of selenite depends on the stage of carcinogenesis, rather than duration of treatment. Evaluation of polyamine levels in hepatoma and surrounding liver tissue showed significant difference in the selenite-treated groups compared with pair-fed control groups. Furthermore, histopathological examination showing remarkable difference between control and treated groups. These results demonstrate that selenite can modulate the development of DEN-induced and PB-promoted rat liver carcinogenesis through a polyamine-dependent mechanism.

Selenium and cancer chemoprevention: hypotheses integrating the actions of selenoproteins and selenium metabolites in epithelial and non-epithelial target cells

The trace element nutrient selenium (Se) discharges its well-known nutritional antioxidant activity through the Se-dependent glutathione peroxidases. It also regulates nuclear factor activities by redox mechanisms through the selenoprotein thioredoxin reductases. Converging data from epidemiological, ecological, and clinical studies have shown that Se can decrease the risk for some types of human cancers, especially those of the prostate, lung, and colon. Mechanistic studies have indicated that the methylselenol metabolite pool has many desirable attributes of chemoprevention, targeting both cancer cells and vascular endothelial cells, whereas the hydrogen selenide pool in excess of selenoprotein synthesis can lead to DNA single strand breaks, which may be mediated by some reactive oxygen species. We propose a new paradigm based on a consideration of the post-initiation biology of avascular early lesion expansion microenvironment, physiochemistry of Se delivery, and the obligatory need for angiogenesis to sustain lesion progression. Our model integrates the roles of selenoproteins and specific Se metabolites to account for cancer risk reduction or enhancement. For future studies, speciation (profiling) methods for Se metabolites and for Se forms in foods and supplements are much needed for hypothesis testing and for the development of mechanism-based Se status markers for cancer prevention. Randomized cancer prevention trials are necessary to test the efficacy of methyl selenium compounds.

Chemical forms of selenium for cancer prevention

Cancer is becoming an increasingly significant disease worldwide. Currently, more than 7 million people die each year from cancer. With the existing knowledge, at least one-third of worldwide cancer cases could be prevented. Searching for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to the control and prevention of cancer. To date, the use of the micronutrient selenium (Se) in human clinical trials is limited, but the outcome indicates that Se is among the most promising agents. Although it is convenient to describe the effects of Se in terms of the element, it must always be kept in mind that the chemical form of Se and the dose are determinants of its biological activities. Hyphenated techniques based on coupling chromatographic separation with inductively coupled plasma mass spectrometric (ICP-MS) detection are now established as the most realistic and potent analytical tools available for real-life speciation analysis. These speciation investigations provide evidence that the Se compounds, which can generate monomethylated Se (e.g., Se-methylselenocysteine and methylseleninic acid), are more efficacious than other Se compounds because of their chemoprevention activity.

Synergic activity of selenium and probiotic bacterium Enterococcus faecium M-74 against selected mutagens in Salmonella assay

Concentrated extracts of MRS (De Man-Rogosa-Sharpe) media in which probiotic bacterium Enterococcus faecium strain M-74 was grown exerted different antimutagenic activity against ofloxacin-, N-methyl, N'-nitro-N-nitrosoguanidine- and sodium 5-nitro-2-furylacrylate-induced mutagenicity in Salmonella typhimurium assay depending on the presence (+Se) or absence of disodium selenite pentahydrate (-Se). The antimutagenicity of MRS(+Se) extract was higher than that of MRS(-Se) extract. Selenium enhanced also the antimutagenic effect of both live and killed cells of E. faecium M-74, respectively. The live bacteria decreased the mutagenicity of selected substances more than killed cells. Synergic activity of selenium with the bacterium was also manifested.

Mechanisms of mammary cancer chemoprevention by organoselenium compounds

Searching for optimal diets and for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to finding ways to control and prevent cancer. To date, the use of the micronutrient selenium in human clinical trials is limited but the outcome of these investigations indicates that selenium is one of the most promising agents. Data presented in this mini-review indicate that the dose and the form (structure) in which selenium is used are the most critical determinants of success in future clinical trials. The focus of this mini-review is on the mechanisms of mammary cancer chemoprevention by organoselenium compounds. Among the naturally occurring organoselenium compounds, Se-Methylselenocysteine is more efficacious than the most extensively studied forms, such as selenomethionine. However, we showed that synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal side effects by structural modifications; it is evident that synthetic agents are superior to the inorganic selenite, naturally occurring selenium compounds and their sulfur-containing analogs. We have demonstrated that 1,4-phenylenebis (methylene) selenocyanate (p-XSC) and its putative metabolite glutathione conjugate (p-XSeSG) are highly promising agents in the chemoprevention of mammary carcinogenesis in the 7,12-dimethylbenz[a]anthracene (DMBA)-rat mammary tumor model system. Both compounds inhibit the initiation phase of carcinogenesis by inhibiting DMBA-DNA adduct formation in the target organ in vivo. cDNA microarray analysis indicates that both selenium compounds alter genes in a manner that leads to inhibition of cell proliferation and induction of apoptosis; modulation of apoptosis and cell proliferation can account for chemoprevention during the post-initiation phase of mammary carcinogenesis. Using a rat mammary cancer cell line, we compared p-XSC and p-XSeSG as inhibitors of cell proliferation; depending on the selenium dose and time point selected, p-XSC was comparable to or better than p-XSeSG. Collectively, the results described here, suggest that the molecular targets modulated by organoselenium compounds are highly useful indicators of success in clinical cancer chemoprevention trials.

Enhanced 7-Ethyl-10-hydroxycamptothecin (SN-38) Lethality by Methylselenocysteine Is Associated with Chk2 Phosphorylation at Threonine-68 and Down-Regulation of Cdc6 Expression

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.

Selenium and its relationship to cancer: an update

Selenomethionine (Semet) is the major seleno-compound in cereal grains and enriched yeast whereas Se-methylselenocysteine (SeMCYS) is the major seleno-compound in Se-accumulator plants and some plants of economic importance such as garlic and broccoli exposed to excess Se. Animals can metabolize both Semet and SeMCYS. Epidemiological studies have indicated an inverse relationship between Se intake and the incidence of certain cancers. Blood or plasma levels of Se are usually lower in patients with cancer than those without this disorder, but inconsistent results have been found with toenail-Se values and the incidence of cancer. There have been eight trials with human subjects conducted on the influence of Se on cancer incidence or biomarkers, and except for one, all have shown a positive benefit of Se on cancer reduction or biomarkers of this disorder. This is consistent with about 100 small-animal studies where Se has been shown to reduce the incidence of tumours in most of these trials. Se-enriched yeast is the major form of Se used in trials with human subjects. In the mammary-tumour model, SeMCYS has been shown to be the most effective seleno-compound identified so far in reduction of tumours. Several mechanisms have been proposed on the mechanism whereby Se reduces tumours. Even though SeMCYS was shown to be the most effective seleno-compound in the reduction of mammary tumours, it may not be the most effective seleno-compound for reduction of colon tumours.

Antiproliferative effect of 1,4-phenylenebis(methylene)selenocyanate (p-XSC) on colonic epithelium of patients with adenomatous polyps in vitro

We have consistently shown that the organoselenium compound 1,4-phenylenebis(methylene)selenocyanate (p-XSC) is a superior cancer chemopreventive agent and less toxic than selenite or certain naturally-occurring selenoamino acids. To elucidate the effects of p-XSC on human colonic mucosa, biopsies from endoscopically normal sigmoid colon of 30 patients with adenomatous polyps were incubated with p-XSC at concentrations of 1, 2 and 5 micromol/l dissolved in dimethylsulphoxide (DMSO). Biopsies incubated with DMSO or pure culture medium served as a control. Proliferating cells were labelled by bromodeoxyuridine immunohistochemistry and the labelling index (LI) was computed. Upper crypt labelling index (LI of crypt compartments 4+5) and Phih value, which are both discriminators of the expansion of the proliferative zone, were significantly lower after incubation with 1 and 5 micromol/l p-XSC, respectively (LI 4+5: 0.8 and 1.0; Phih value: 2.1 and 2.4), as compared with DMSO (LI 4+5: 3.6 and 4.5; Phih value: 7.0 and 8.3) or culture medium (LI 4+5: 3.3 and 4.5; Phih value: 7.2 and 8.1) (P<0.005 and P<0.05 by Friedman's block test). A trend towards lower levels of LI 4+5 (P=0.059) and Phih value (P=0.075) were seen after 2 micromol/l p-XSC incubation compared with DMSO. Since hyperproliferation of colonic crypt cells with expansion of the proliferative zone is regarded as a biomarker of increased cancer risk, the antiproliferative effects of p-XSC especially on upper crypt LI and Phih value may indicate a possible protective effect of this organoselenium compound in the prevention of human colon cancer development.

The antioxidant role of selenium and seleno-compounds

Selenium (Se) is an essential trace element for animals and humans that is obtained from dietary sources including cereals, grains and vegetables. The Se content of plants varies considerably according to its concentration in soil. Plants convert Se mainly into Se-methionine (Se-Met) and incorporate it into protein in place of methionine (Met). Selenocystine (Se-Cys), methyl-Se-Cys and gamma-glutamyl-Se-methyl-Cys are not significantly incorporated into plant protein and are at relatively low levels irrespective of soil Se content. Higher animals are unable to synthesize Se-Met and only Se-Cys was detected in rats supplemented with Se as selenite. Renal regulation is the mode by which whole body Se is controlled. Se is concentrated in hair and nail and it occurs almost exclusively in organic compounds. The potentiating effect of Se deficiency on lipid peroxidation is enhanced in some tissues by concurrent deficiency of copper or manganese. In the in vitro system, the chemical form of Se is an important factor in eliciting cellular responses. Although the cytotoxic mechanisms of selenite and other redoxing Se compounds are still unclear, it has been suggested that they derive from their ability to catalyze the oxidation of thiols and to produce superoxide simultaneously. Selenite-induced cytotoxicity and apoptosis in human carcinoma cells can be inhibited with copper (CuSO(4)) as an antioxidant. High doses of selenite result in induction of 8-hydroxydeoxyguanosine (8-OHdG) in mouse skin cell DNA and in primary human keratinocytes. It may cause DNA fragmentation and decreased DNA synthesis, cell growth inhibition, DNA synthesis, blockade of the cell cycle at the S/G(2)-M phase and cell death by necrosis. In contrast, in cells treated with methylselenocyanate or Se methylselenocysteine, the cell cycle progression was blocked at the G(1) phase and cell death was predominantly induced by apoptosis.

Multiorgan sensitivity to anticarcinogenesis by the organoselenium 1,4-phenylenebis(methylene)selenocyanate

The data in this report clearly indicate that the form (structure) in which selenium is used is the most critical determinant of success in future clinical trials. Synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal toxic side effects by structural modifications. We demonstrated that 1,4-phenylenebis(methylene)selenocyanate is a powerful chemopreventive agent against the development of experimental colon, mammary, lung, and oral carcinogenesis. On the basis of metabolism studies of organoselenium compounds and those reported in the literature, our working hypothesis is that aromatic selenol intermediates are important entities in cancer chemoprevention. In addition, we suggest that 1,4-phenylenebis(methylene)selenocyanate not only serves as a chemopreventive agent, but it may be valuable in preventing metastatic diseases in future studies in the clinic.

Molecular mechanisms of cancer prevention by selenium compounds

Selenium compounds that are chemopreventive in animal models inhibit cell growth and induce apoptosis in vitro, and this could explain how they reduce the outgrowth of tumor cells in vivo. Our recent work has shown that primary cultures of oral carcinoma biopsies are significantly more sensitive than normal oral mucosa cultures to induction of apoptosis by a natural selenium metabolite [selenodiglutathione (SDG)], and this is associated with induction of Fas ligand, a well-known mediator of apoptosis in other contexts, and activation of so-called stress kinase signaling pathways, particularly the Jun NH2-terminal kinase (JNK). Heme oxygenase, another marker of stress responses, is also induced by selenite and SDG. The selective activation of the Fas pathway in carcinomas could be responsible directly for their destruction by apoptosis or target them for attack by immunologic responses. In contrast, although the potent pharmacological selenium chemopreventive agent 1,4-phenylenebis(methylene)selenocyanate (p-XSC) also induces Fas ligand, heme oxygenase, and stress kinase pathways, apoptosis/Fas induction is not so strongly JNK-dependent and p-XSC does not show tumor selectivity. These differences in mechanism between SDG and p-XSC may be due to the manner in which they induce redox changes in the cells, since although the effects of SDG and p-XSC are prevented by antioxidants such as glutathione or N-acetylcysteine, hydroxyl radical scavengers such as mannitol or pyrrolidine dithiocarbamate only protect against the effects of p-XSC.

Impact of selenium and cancer-prevention findings on the nutrition-health paradigm

Evidence that selenium supplementation can reduce cancer risk is difficult to incorporate in nutrition thinking in which "nutritional essentiality" is the central concept. That concept, which defines nutrient need in terms of indispensability in diets and irreplaceable function in preventing specific deficiency disorders, was not developed to accommodate the function of a nutrient in reducing the risk to chronic disease, particularly when that function is not obligate but may be among several involved in maintaining good health. The findings of Clark et al. (JAMA 276, 1957-1963, 1996; Br J Urol 81, 730-734, 1998) suggest that selenium intakes of approximately twice the levels of the new Recommended Dietary Allowance or more can have such beneficial health effects. Because these intakes are above those required to support its accepted essential biochemical functions and because the maintenance of good general health as much as the prevention of specific deficiency disorders is the goal of public health, it is appropriate to reassess the nutritional essentiality paradigm. This discussion of the development and outcomes of the clinical intervention trials of Larry Clark and colleagues is presented in light of these issues.

Se-methylselenocysteine induces apoptosis through caspase activation and Bax cleavage mediated by calpain in SKOV-3 ovarian cancer cells

Se-methylselenocysteine (Se-MSC) is a potent chemopreventive agent in many test systems and has been shown to inhibit tumor promotion and induce apoptosis, but its mechanism of action is still not well understood. The present study was designed to assess the mechanism of Se-MSC on the induction of apoptosis in SKOV-3 ovarian cancer cells. Se-MSC displayed strong inhibitory effects on cell proliferation and viability of SKOV-3 cells in dose and time dependent manners and induced apoptosis. Investigation of the mechanism of Se-MSC-induced apoptosis revealed that treatment with Se-MSC produced morphological features of apoptosis and DNA fragmentation. This was associated with caspase-3 activation and cleavage of poly(ADP-ribose) polymerase and phospholipase C-gamma1 proteins. However, SKOV-3 cells treated with Se-MSC did not demonstrate cytochrome c accumulation in the cytosol during apoptosis induction. Pretreatment of cells with the caspase inhibitors (z-VAD-fmk and DEVD-CHO) prevented Se-MSC-induced apoptosis. These results suggested that Se-MSC induces apoptosis through cytochrome c-independent caspase-3 activation in SKOV-3 cells. In late stage of apoptosis, p18kDa fragment of Bax was generated with the down-regulation of the expressions of survivin, X-linked inhibitor of apoptosis protein, and human inhibitor of apoptosis protein 1 following Se-MSC treatment, suggesting that the modulation of Bax and IAP (inhibitors of apoptosis) family proteins play some role in Se-MSC-mediated apoptosis. Pre-treatments of z-VAD-fmk and the calpain inhibitor, calpeptin inhibited Bax cleavage. These results suggested that Bax cleavage is mediated by calpain, and calpain activation may be a caspase-dependent one. Taken together, the chemopreventive effects of Se-MSC may be related in part to the caspase-3 activation, the down-regulation of IAP family proteins, and Bax cleavage mediated by caspase-dependent calpain activation.

Selenocompounds in plants and animals and their biological significance

There are several selenocompounds in tissues of plants and animals. Selenate is the major inorganic selenocompound found in both animal and plant tissues. Selenocysteine is the predominant selenoamino acid in tissues when inorganic selenium is given to animals. Selenomethionine is the major selenocompound found initially in animals given this selenoamino acid, but is converted with time afterwards to selenocysteine. Selenomethionine is the major selenocompound in cereal grains, grassland legumes and soybeans. Selenomethionine can also be the major selenocompound in selenium enriched yeast, but the amount can vary markedly depending upon the growth conditions. Se-methylselenocysteine is the major selenocompound in selenium enriched plants such as garlic, onions, broccoli florets and sprouts, and wild leeks.

Selenomethionine induction of DNA repair response in human fibroblasts

Selenium compounds have a long history in chemoprevention of mammary and colon cancers in rodent models. Selenium compounds are in current clinical trials, having shown promise in prevention of prostate and other human cancers. In human tissues, it has been estimated that each cell sustains approximately 10 000 potentially mutagenic (if not repaired) lesions per day due to endogenous DNA damage. Almost no studies have addressed the potential for selenium compounds to induce DNA repair, a potential mechanism for their cancer-preventive actions. We show that selenium in the form of selenomethionine induces a DNA repair response in normal human fibroblasts in vitro, and protects cells from DNA damage. We show a possible mechanism for the inducible DNA repair response, in which enhanced repair complex formation was observed in selenomethionine-treated cells.

Catalytic oxidation of zinc/sulfur coordination sites in proteins by selenium compounds

Zinc/thiolate (cysteine) coordination occurs in a very large number of proteins. These coordination sites are thermodynamically quite stable. Yet the redox chemistry of thiolate ligands confers extraordinary reactivities on these sites. The significance of such ligand-centered reactions is that they affect the binding and release of zinc, thus helping to distribute zinc, and perhaps controlling zinc-dependent cellular events. One new aspect focuses on the thiolate ligands of zinc as targets for the redox action of selenium compounds. A distinctive feature of this chemistry is the capacity of selenols to catalyze the oxidation of zinc/thiolate sites. We here use a chromophoric compound, 2-nitrophenylselenocyanate, to investigate its reaction mechanism with the zinc/thiolate clusters of metallothionein, a protein that is a cellular reservoir for zinc and together with its apoprotein, thionein, is involved in zinc distribution as a zinc donor/acceptor pair. The reaction is particularly revealing as it occurs in two steps. A selenenylsulfide intermediate is formed in the fast oxidative step, followed by the generation of 2-nitrophenylselenol that initiates the second, catalytic step. The findings demonstrate the high reactivity of selenium compounds with zinc/thiolate coordination sites and the potent catalytic roles that selenoproteins and selenium redox drugs may have in affecting gene expression via modulation of the zinc content of zinc finger proteins.

Se-methylselenocysteine induces apoptosis through caspase activation in HL-60 cells

Apoptosis, a programmed process of cell suicide, has been proposed as the most plausible mechanism for the chemopreventive activities of selenocompounds. In our study, we found that Se-methylselenocysteine (MSC) induced apoptosis through caspase activation in human promyelocytic leukemia (HL-60) cells. Measurements of cytotoxicity, DNA fragmentation and apoptotic morphology revealed that MSC was more efficient at inducing apoptosis than selenite, but was less toxic. Moreover, MSC increased both the apoptotic cleavage of poly(ADP-ribose) polymerase (PARP) and caspase-3 activity, whereas selenite did not. We next examined whether caspases and serine proteases are required for the apoptotic induction by MSC. A general caspase inhibitor, z-VAD-fmk, dramatically decreased cytotoxicity in MSC-treated HL-60 cells and several other apoptotic features, such as, caspase-3 activation, the apoptotic DNA ladder, TUNEL-positive staining and the DNA double-strand break. Interestingly, a general serine protease inhibitor, AAPV-cmk, also effectively inhibited MSC-mediated cytotoxicity and apoptosis. These results demonstrate that MSC is a selenocompound that efficiently induces apoptosis in leukemia cells and that proteolytic machinery, in particular caspase-3, is necessary for MSC-induced apoptosis. On the other hand, selenite-induced cell death could be derived from necrosis rather than apoptosis, since selenite did not significantly induce several apoptotic phenomena, including the activation of caspase-3.

Molecular targets for selenium in cancer prevention

Mounting evidence reveals that selenium is a dietary constituent with anticarcinogenic and antitumorigenic properties. Various forms of selenium appear to be effective in bringing about these effects, although preclinical studies suggest that differences may arise as the quantity provided is reduced. The literature also documents the greater sensitivity of neoplastic cells to selenium than their nonneoplastic counterparts. Unfortunately, the minimal amount needed to bring about a positive effect in humans remains elusive. If there is a positive response to exaggerated intakes, it will likely be dependent on many factors, including the consumption of other dietary constituents, as well as variation in a host of genetic pathways involved with cancer. Although the biological basis of the reduction in cancer risk ascribed to selenium remains to be established, its consistency in retarding various experimentally induced tumors and suppressing the growth of various types of neoplasms in vitro and in vivo suggests that several mechanisms are involved. Depressed carcinogen bioactivation, reduced cell proliferation, and increased apoptosis raise the possibility that selenium works at a number of specific molecular targets involved with the cancer process. This review will focus on molecular targets involved with cell proliferation and apoptosis as possible mechanisms by which selenium might alter the cancer process.

Chemoprevention of lung tumorigenesis induced by a mixture of benzo(a)pyrene and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone by the organoselenium compound 1,4-phenylenebis(methylene)selenocyanate

We evaluated the chemopreventive efficacy of the organoselenium compound 1,4-phenylenebis(methylene)selenocyanate (p-XSC) against the development of tumors of the lung and forestomach induced by a mixture of benzo(a)pyrene (B(a)P) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), two of the major lung carcinogens present in tobacco smoke. A/J mice (20 mice/group) were given intragastric doses of a mixture of B(a)P (3 micromol/mouse) and NNK (3 micromol/mouse) in cottonseed oil (0.1 ml) once a week for eight consecutive weeks. Mice were fed either AIN-76A control diet or control diet containing p-XSC (10 ppm selenium), either during or after carcinogen administration. Dietary p-XSC significantly reduced lung tumor multiplicity, regardless of whether it was given during or after carcinogen administration. p-XSC was also an effective inhibitor of tumor development in the forestomach. To provide some biochemical insights into the protective role of p-XSC, its effect on selected phase I and II enzyme activities involved in the metabolism of NNK and B(a)P was also examined in vivo in this animal model. Dietary p-XSC significantly inhibited the activities of the phase I enzymes, methoxyresorufin O-dealkylase (MROD) and N-nitrosodimethylamine N-demethylase (NDMAD), in mouse liver, but it had no effect on ethoxyresorufin O-dealkylase (EROD), pentoxyresorufin O-dealkylase (PROD), and erythromycin N-demethylase (ERYTD). Total glutathione S-transferase (GST) enzyme activity, as well as GST-pi and GST-mu enzyme activities, were significantly induced by dietary p-XSC in both the lung and liver. Glutathione peroxidase (GPX) activity was also induced by p-XSC in mouse lung, but not in the liver. Dietary p-XSC had no effect on selenium-dependent glutathione peroxidase (GPX(Se)), GST-alpha, and UDP-glucuronosyl transferase (UDPGT) enzyme activities in either the lung or the liver. These studies suggest that the chemopreventive efficacy of p-XSC, when fed during carcinogen administration, may be, in part, due to the inhibition of certain phase I enzymes involved in the metabolic activation of these carcinogens, and the induction of specific phase II enzymes involved in their detoxification. The mechanisms that account for the effect of p-XSC when fed after carcinogen administration remain to be determined.

Monomethyl selenium--specific inhibition of MMP-2 and VEGF expression: implications for angiogenic switch regulation

Previous work suggested that antiangiogenic activity may be a novel mechanism contributing to the cancer chemopreventive activity of selenium (Se). Because methylselenol has been implicated as an in vivo active chemopreventive Se metabolite, experiments were conducted to test the hypothesis that this metabolite pool might inhibit the expression of matrix metalloproteinase-2 (MMP-2) by vascular endothelial cells and of vascular endothelial growth factor (VEGF) by cancer epithelial cells, two proteins critical for angiogenesis and its regulation. In human umbilical vein endothelial cells (HUVECs), zymographic analyses showed that short-term exposure to methylseleninic acid (MSeA) and methylselenocyanate (MSeCN), both immediate methylselenol precursors, decreased the MMP-2 gelatinolytic activity in a concentration-dependent manner. In contrast, Se forms that enter the hydrogen selenide pool lacked any inhibitory effect. The methyl Se inhibitory effect on MMP-2 was cell dependent because direct incubation with Se compounds in the test tube did not result in its inactivation. Immunoblot and enzyme-linked immunosorbent assay analyses showed that a decrease of the MMP-2 protein level largely accounted for the methyl Se-induced reduction of gelatinolytic activity. The effect of MSeA on MMP-2 expression occurred within 0.5 h of exposure and preceded MSeA-induced reduction of the phosphorylation level of mitogen-activated protein kinases (MAPKs) 1 and 2 (approximately 3 h) and endothelial apoptosis (approximately 25 h). In addition to these biochemical effects in monolayer culture, MSeA and MSeCN exposure decreased HUVEC viability and cell retraction in a three-dimensional context of capillary tubes formed on Matrigel, whereas comparable or higher concentrations of selenite failed to exert such effects. In human prostate cancer (DU145) and breast cancer (MCF-7 and MDA-MB-468) cell lines, exposure to MSeA but not to selenite led to a rapid and sustained decrease of cellular (lysate) and secreted (conditioned medium) VEGF protein levels irrespective of the serum level (serum-free medium vs. 10% fetal bovine serum) in which Se treatments were carried out. The concentration of MSeA required for suppressing VEGF expression was much lower than that needed for apoptosis induction. Taken together, the data support the hypothesis that the monomethyl Se pool is a proximal Se for inhibiting the expression of MMP-2 and VEGF and of angiogenesis. The data also indicate that the methyl Se-specific inhibitory effects on these proteins are rapid and primary actions, preceding or independent of inhibitory effects on mitogenic signaling at the level of MAPK1/2 and on cell growth and survival.